The death of Khmer Rouge co-founder Ieng Sary earlier this month renewed fears the elderly accused may not live to see the tribunal's verdicts.

But after speaking with medical experts, Judge Nil Nonn from the United Nations backed war crimes court, has ruled Nuon Chea remains capable of meaningfully participating in his own defence.

The 86-year-old is the oldest surviving leader of the genocidal communist regime and is on trial with former Khmer Rouge head of state Khieu Samphan.

The pair deny the war crimes and genocide charges brought against them.

Ieng Sary's widow Ieng Thirith, the regime's former social affairs minister, was freed in September after being deemed unfit for trial due to dementia.

Nuon Chea has suffered a number of illnesses, including high blood pressure, acute bronchitis and heart disease, prompting his defence team to argue that their client was too weak to stand trial.

The tribunal has been dogged by funding shortages since it was set up in 2006 and was hit by a strike by local staff earlier this month over unpaid wages.

Led by "Brother Number One" Pol Pot, who died in 1998, the Khmer Rouge from 1975-79 wiped out nearly a quarter of Cambodia's population through starvation, overwork or execution in a bid to create an agrarian utopia.